The present invention relates to the field of oil and gas production methods and apparatus, and more particularly to the field of tubular conduits used in the production of oil and gas from offshore locations. More particularly still, the present invention relates to the field of high pressure risers for tying an existing well at the bottom of a body of water back to a platform at the surface.
In the production of oil and gas from offshore locations, it is often advantageous to employ a platform or similar structure at the surface of the body of water above the field to be developed in order to centralize the drilling and/or production operations for a plurality of wells in the vicinity of the platform. When an offshore field development program includes the use of such a platform, frequently the most economical approach is to predrill all or a substantial portion of the wells prior to installation of the platform, due primarily to the long lead time required for manufacturing and installing an offshore platform. Typically, the wells that are predrilled are left at the mudline ready for completion and in a condition which allows the operators to tie them back to the platform when the time comes. This means that the several concentric strings of progressively smaller diameter, but longer, casing comprised in the particular casing program are already run, hung off in the wellhead, and cemented in place, and the wellhead may be capped at its upper end with, for example, a temporary abandonment cap. As an example, such a temporarily abandoned well may have a 30" conductor pipe, a 20"" surface casing, and intermediate casing strings of 133/8" and 95/8 (or 103/4") OD already installed. When the platform is installed and ready for service, the predrilled wells are tied back to the platform and completed, and the remaining wells in the program, if any, may be drilled from the platform using, for example, conventional land-type equipment and techniques.
According to conventional practice, the predrilled wells are typically tied back to the surface platform from the wellhead at the sea floor by running and setting a plurality of concentric risers from the platform to selected casing strings, and the outermost riser is generally tied back to the wellhead for structural integrity. The risers provide access from the platform to the corresponding annuli between the respective casing strings in order to monitor annulus pressures and to facilitate the production, workover, or other operations conducted from the platform. Each of the risers of the plurality of concentric risers is typically run, stabbed in, and sealed at the wellhead separately, thus requiring a plurality of sometimes difficult, usually time-consuming, and sometimes ineffective remote stab-in and seal energizing procedures. With the high hourly costs associated with operating from an offshore platform, the cost of individually running and setting the plurality of concentric risers can be great.
Recently, a breakthrough in the design of offshore wellhead equipment has occurred which permits the respective annuli in the wellhead to be accessed through longitudinally extending fluid passageways disposed in the walls of the wellhead housing which communicate at one end with the respective annuli and at the other end with a space above the upper surface of the wellhead housing. See U.S. Pat. No. 4,887,672, issued Dec. 19, 1989, to the present Applicant and owned by the assignee of the present application (hereinafter referred to as "the '672 patent"), the disclosure of which is incorporated herein by reference. The '672 patent did not address, however, an adequate solution to the problems referred to above in connection with the traditional high cost of running, stabbing in, and sealing a plurality of concentric risers from the platform to the wellhead. Nevertheless, the breakthrough embodied in the '672 patent has set the stage for the present invention, which permits the simple, effective, efficient, and economical communication of the respective annulus access passageways or bores in a wellhead housing of that type to the surface while eliminating the need for the plurality of concentric risers of the prior art with their attendant high installation costs.